Corrosion is a phenomenon that objects such as metals, concretes and woods suffer from chemical actions or electrochemical actions of the ambient environmental medium. According to relevant statistics, the economic losses due to corrosion around the world are 700 billion to 1 trillion dollars. It is about six times more than the sum total of economic losses due to natural hazards such as earthquake, flood, and typhoon etc. The economic losses due to corrosion in China are 800 billion, accounting for around 3% of GDP. In order to slow down the corrosion rate, many techniques have been developed.
Nowadays, there are several fundamental anti-corrosion coatings mainly including epoxy, polyurethane and zinc-rich primer coatings. Among those, the epoxy anti-corrosion coatings have excellent adhesion, superior mechanical property, good alkali resistance and chemical medium corrosion resistance, and high content of solid component. However, the coating films formed from the epoxy anti-corrosion coatings exhibit high hardness and poor flexibility at the early stage. The bituminous and chromate epoxy anti-corrosion coatings are greatly destructive to environment and harmful to human health. Polyurethane coatings have advantages such as strong adhesion, water resistance, oil resistance, solvent resistance, corrosion resistance, good elasticity, high strength, and low temperature resistance, and the formed coating films is bright, stiff, flexible, thick, wear-resistant and scratch-resistant. However, they have poor gloss and colour retention, strong irritation and toxicity, poor stability and inconvenient use. The zinc-rich primer coatings have shielding effect, electrochemical protection effect, self-healing of coating film, and inactivation effect. However, they achieve anti-corrosion features at the expense of sacrificing zinc powder, and they generate a large amount of zinc oxide vapour during welding, thereby harming life health of occupational workers.
Chinese patent publication No. CN101407690A discloses a two-component epoxy zinc-rich primer and the preparation method thereof. Component A consists of, by weight parts, 14 parts of epoxy resin, 54-71 parts of zinc powder, 4-20 parts of filler, 1-3 parts of anti-settling agent, and 9 parts of solvent. Component B comprises, by mass percentage contents, 40% of curing agent and 60% of solvent. The mass ratio of component A to component B is 8:1-15:1. The coating according to this invention has only 3 days, i.e. 72 hours of the time of salt fog resistance, and it requires 54-71 parts of zinc powder.
In current industrial standard HG/T3668-2009, the time of salt fog resistance for organic zinc-rich primer is indicated, i.e. non-volatile components (%) are ≥70. The time of salt fog resistance is required for 600 h if the contents of metal zinc in the non-volatile components (%) are ≥80; and the time of salt fog resistance is required for 200 h if the contents of metal zinc in the non-volatile components (%) are ≥60.
Corrosion will occur when the surface of metals is contacted with air, water or other substances. The fundamental method for preventing corrosion is to coat metals with coating materials to shield the surface of metals. However, the currently used coating materials have their own limitations.
Graphene is a new carbonaceous material, in which the carbon atoms closely packed in a monolayer, and have a dimensional sheet-like structure. It has excellent electrical properties and large diameter-to-thickness ratio (>200) as well as thin sheet-like structure (<1 nm). It is known as the thinnest and strongest material. The arrangement of carbon atoms in graphene is just like a network of chicken paw prints. Each layer is very thin and transparent. 28 football pitches can be covered by using one ounce of it (Graphene: Corrosion-Inhibiting Coating, Dhiraj Prasai et al., ACS NANO, Vol. 6, No. 2, 2012: pp. 1102-1108). Dhiraj Prasai et al. indicated that the corrosion of underneath metal could be inhibited by utilizing graphene having atomic lamina as protective coating. However, the graphene in this report is grown through chemical vapour deposition. Therefore, this method only remains in the phase of theoretical research. In addition, this method is enormously expensive, and difficult in manufacturing.
Chinese patent publication No. CN102604533A discloses an anti-corrosion coating based on a composite material of polyaniline and graphene, and the preparation method thereof. The anti-corrosion coating consists of, by weight parts, 20-90 parts of film forming substance, 0.1-20 parts of composite material of polyaniline and graphene, 1-10 parts of pigment, 1-10 parts of filler, 0.1-3 parts of anti-sagging agent, 0.1-4 parts of dispersing agent, 0.01-2 parts of leveling agent, 0.01-2 parts of antifoaming agent, and 1-50 parts of solvent. According to this invention, graphene, with high diameter-to-thickness ratio and good flexibility, can provide superior physical anti-corrosion effect, while polyaniline absorbed on the surface of graphene result in electrochemical anti-corrosion effect. The mechanism of synergistic effect of them allows the coating having superior metal corrosion protection performance. The anti-corrosion coating of this invention can achieve the activation and corrosion inhibition of metal substrate while avoid the employment of heavy metals such as chromium that can result in environmental pollution. The coating in this invention is easy to prepare and process, low-cost, and suitable for industrial production. However, during the preparation of the anti-corrosion coating, as claimed in Chinese patent publication No. CN102604533A, the step of adding graphene is complex and require supporting solutions such as an oxidizing agent.
Therefore, it is highly desired to develop an environmentally friendly zinc-rich epoxy anti-corrosion coating, which can generate a relatively thin coating film with good flexibility, high hardness, low zinc content, and long time of salt fog resistance as well as acid and alkali resistance in the art. At the same time, the processing of the epoxy resin coating should be simple.